The speakers probably sounded state-of-the-art when they were new, but I suspect that technology has moved on since then.
There's a fella not far from me selling a pair of Mission 760i's on eBay, but I've heard that Mission speakers can sound bright or shrill -- and I prefer a sound that's rich and smooth.
Does anyone have any experience of Mission bookshelf speakers?
No such thing with the DM4s as 'technology' - they use the same technology as a modern speaker - moving coils, cones and a cross-over. The capacitors may have dried out a bit and and would benefit from replacement. The rest of the speaker will have hardly changed except maybe the cone surround. Foam perishes, rubber hardens but in most cases jack shit happens.
The HF units in those speakers are worth quite a bit btw.
As for Mission sounding bright? Never heard of that, ever. More likely a source issue, Rotel had their CD player line out at 3V whereas most other manufacturers were 1.5V instantly giving Rotels a bigger sound.
760i is a good small speaker with a lot of low end from the design. But they are small and probably not as efficient as the DM4.
I would look at replacing the capacitors on the DM4 and leaving it at that.
*edit: looking at your CD player, that would be your weakest link - lower model always sounded a bit naff compared to their more expensive siblings. Look on ebay for rosewood side CD players of old. Their sound from hifi specific laser mechs seems much better than later CD ROM based machines. More time seemed to have been spent on D to A conversion. Machines between about 1986 and 1992 appear to be better than later models. Of course they are getting on a bit but I've always seemed to have found older machines better than their newer equivalents.
Higher end modern still seems to do well but nobody seems to take into account how the music was mastered anymore. There are some awful CD's about and listening to MP3/4 downloads, these vary in mastering quality just as much if not more.